There's a great book called Start Small, Stay Small: A Developer's Guide to Launching a Startup by Rob Walling [1]. It's by far my favorite book I've read on this subject and made me reconsider my approach of trying to monetize a side project. It was written in 2010 but is still highly relevant and recommended here from time to time. (There's similar free content on his blog as well.)

One important idea from the book is the distinction of side project/product confusion:

> A project is a software application that you build as a fun side project. The code is fun to write because you’re not concerned about quality and performance, and the end result is a neat little application that likely isn’t of use to many people.

> A product is a project that people will pay money for. In other words, it’s a project that has a market (a group of people who want to buy it). Without a market, a software application is just a project.

I think it's important to start in the right place here. Both approaches are fun but they have opposing goals. If you want to build a product that makes money, start with the market. If you want to build a side project... that's great, just keep in mind that when a side project tries to tack on "and make money" later, it mostly doesn't work.

[1]: http://www.startupbook.net/